The invention relates to preparing a steel wire for subsequent stress testing and the apparatus for accomplishing the same. More particularly, this invention relates to providing a means by which steel cord, as is found internally of a pneumatic tire, can be tensile tested.
It is often desirable for engineers to determine stress data when metallic wire is subjected to a load. For example, tire engineers are quite interested in the stresses which fabric reinforcing cord encounters in an operating tire so that efficient tire design and performance is assured. Such data is even more critical when the steel reinforced radial ply tire is being designed because it is the deformation characteristics of the steel reinforced belt when subjected to loads that gives the radial tire its advantageous feature of low tread squirm thereby promoting low rate of wear.
Heretofore two general approaches have been taken in an attempt to obtain the stress data from a cord in a tire. In one approach the tire is conventionally built and then a layer of rubber from the inside of the tire buffed off to expose the first ply cords to which a strain gauge is applied. This procedure is somewhat useful for mapping strains in the crown to bead area of the tire and for following progressive strain states attained during inflation and during passage of the area through the tire footprint over an obstacle. However, this approach has not been totally satisfactory in that only the cords on the inner tire surface are available for the test thereby making it impossible to gather data from cords in other plies or in other areas of the tire. Further, the back side of the cord is not available for attachment of a compensating gauge, which resulted in test data which inseparably included both tensile strains and bending strains.
The other approach taken is to first apply the strain gauge to the cord and then build that cord into a test tire. While the building of the tire with cords having gauges therein is not without its problems, even more severe are the problems of properly applying the gauge to the cord prior to implant. One method which has been suggested involves splicing the cord and mechanically affixing the strain gauge to the ends of the splice. But in so doing, usually the ends of the cord must be bent sharply and/or crimped to effect the connection which not only results in inducing false stresses into the cord and promoting premature cord failure, but also results in a high probability of cord slippage under load, that is, slippage at the junctions between the gauge and the cord. In addition, the fact that the cord is rendered discontinuous also adversely affects the test results in that the stress characteristics of the gauge itself must be considered. Further, in some types of mechanical attachments of the strain gauge to the spliced cord, for example, when the ends of the wires are cemented into a hollow metallic cylinder carrying the gauge, a stiffening of the cord over a considerable distance is encountered which induces error into a subsequent tensile test.